Human respiratory syncytial virus (RSV) infects nearly everyone worldwide early in life and is responsible for considerable mortality and morbidity. In the United States alone, RSV is responsible for 75,000-125,000 hospitalizations yearly, and conservative estimates indicate that RSV is responsible worldwide for 64 million pediatric infections and 160,000 or more pediatric deaths each year. Another notable feature of RSV is that severe infection in infancy frequently is followed by lingering airway dysfunction, including a predisposition to airway reactivity, that in some individuals lasts for years and can extend into adolescence and beyond. RSV infection exacerbates asthma and may be involved in initiating asthma.
RSV is a member of the Paramyxoviridae family and, as such, is an enveloped virus that replicates in the cytoplasm and matures by budding at the host cell plasma membrane. The genome of RSV is a single, negative-sense strand of RNA of 15.2 kilobases that is transcribed by the viral polymerase into 10 mRNAs by a sequential stop-start mechanism that initiates at a single viral promoter at the 3′ end of the genome. Each mRNA encodes a single major protein, with the exception of the M2 mRNA that has two overlapping open reading frames (ORFs) encoding two separate proteins M2-1 and M2-2. The 11 RSV proteins are: the RNA-binding nucleoprotein (N), the phosphoprotein (P), the large polymerase protein (L), the attachment glycoprotein (G), the fusion protein (F), the small hydrophobic (SH) surface glycoprotein, the internal matrix protein (M), the two nonstructural proteins NS1 and NS2, and the M2-1 and M2-2 proteins. The RSV gene order is: 3′-NS1-NS2-N-P-M-SH-G-F-M2-L. Each gene is flanked by short conserved transcription signals called the gene-start (GS) signal, present on the upstream end of each gene and involved in initiating transcription of the respective gene, and the gene-end (GE) signal, present at the downstream end of each gene and involved in directing synthesis of a polyA tail followed by release of the mRNA.
The development of RSV vaccines has been in progress since the 1960's but has been complicated by a number of factors. For example, immunization of RSV-naïve infants with inactivated RSV has been shown to prime for enhanced disease upon subsequent natural RSV infection, and studies in experimental animals indicate that disease enhancement also is associated with purified RSV subunit vaccines.
Another obstacle to immune protection is that RSV replicates and causes disease in the superficial cells of the respiratory airway lumen, where immune protection has reduced effectiveness. Thus, immune control of RSV infection is inefficient and often incomplete, and it is important for an RSV vaccine to be as immunogenic as possible. Another obstacle to RSV vaccines is that the magnitude of the protective immune response is roughly proportional to the extent of virus replication (and antigen production). Thus, the attenuation of RSV necessary to make a live vaccine typically is accompanied by a reduction in replication and antigen synthesis, and a concomitant reduction in immunogenicity, and therefore it is beneficial to identify a level of replication that is well tolerated yet satisfactorily immunogenic.
Another obstacle is that RSV grows only to moderate titers in cell culture and is often present in long filaments that are difficult to purify. RSV can readily lose infectivity during handling. Another obstacle is the difficulty in identifying and developing attenuating mutations. Appropriate mutations must be attenuating in vivo, but should be minimally restrictive to replication in vitro, since this is preferred for efficient vaccine manufacture. Another obstacle is genetic instability that is characteristic of RNA viruses, whereby attenuating mutations can revert to the wild-type (wt) assignment or to an alternative assignment that confers a non-attenuated phenotype. Instability and de-attenuation are particularly problematic for point mutations.
Taking these factors together, there is a need for live attenuated RSV strains that replicate efficiently in vitro, are maximally immunogenic, are satisfactorily attenuated, and are refractory to de-attenuation.